Working off information from a Department of Justice whistle-blower, four Republican lawmakers are seeking information on the use of FBI planes by top DOJ officials, including Attorney General Eric Holder.

"We believe it is important to determine whether use of the FBI's aircraft by Attorney General Holder and other senior Department of Justice officials adheres to relevant statutes and policies, is cost effective, and does not hinder the operational readiness of the FBI," the group wrote in a letter to FBI Director Robert Mueller.

"We are concerned that FBI aircraft are used for extraneous business and personal travel by senior DOJ officials, including the Attorney General," the letter continued.

The group of lawmakers - which included Senate Judiciary ranking member Chuck Grassley (Iowa) and House Judiciary Chairman Lamar Smith (Texas) - said it is concerned that Holder has been one of the offenders.

The letter states: "We heard troubling allegations that the Attorney General is among those who have reserved and used FBI planes for his own travel when aircraft were needed for FBI missions, then upgraded to a larger aircraft owned by a different agency and left the FBI plane sitting idle because he failed to notify the FBI in a timely manner. These allegations were particularly troubling because they suggested the FBI had to lease another plane to ensure the availability of aircraft for FBI operations."

Senate Appropriations Subcommittee on Commerce, Justice and Science ranking member Kay Bailey Hutchison (Texas) and House Appropriations Subcommittee on Commerce, Justice and Science Chairman Frank Wolf (Va.), also signed the letter. Those subcommittees control the DOJ's budget.

A Republican aide to the Senate Judiciary Committee said the allegations came from "a whistle-blower within the Justice Department."

The letter comes after reports that Secretary of Defense Leon Panetta incurred about $870,000 in personal travel to and from his home in California using Department of Defense airplanes.

Republicans in Congress, including Grassley, have already butted heads with Holder over the "Operation Fast and Furious" gun walking investigation.

In June, the Republican-led House voted to hold Holder in contempt of Congress for refusing to turn over documents tied the investigation.

"If true, this travel would place unnecessary budgetary pressure on the FBI and may adversely impact the operational readiness of the FBI's air operations," the letter said. "Although senior officials may reimburse the government for non-official travel on government aircraft, the reimbursement rate does not necessarily make the taxpayer whole for the actual costs incurred by using government aircraft."

The lawmakers are seeking answers to a raft of questions, including what "percentage of aircraft use was for executive transportation, investigative operations, and pilot training/maintenance for fiscal years 2008, 2009, 2010, 2011, and 2012."

With regard to reserving FBI planes, the lawmakers want to know: "How many times has a request for FBI aviation support been unfulfilled because personnel and/or equipment were not available?" and "How many investigative operations had unfulfilled aviation requests regardless of reason?"

The lawmakers are also seeking details of claims that Holder failed to notify the FBI in a timely manner when he decided to change his travel plans.

"If these allegations are true, describe in detail each such instance where the FBI had to lease an additional plane due to Justice Department executive travel," the letter requests.

The lawmakers are also interested in the cost of personal travel by DOJ officials.

"Does the Attorney General reimburse the taxpayer for personal travel expenses when he uses FBI aircraft, like Secretary Panetta does with the DOD?" the letter asks. "If so, at what rate does he reimburse the taxpayer and how does that rate compare with the actual costs of using the aircraft? How much was reimbursed total in each fiscal year: 2008, 2009, 2010, 2011, and 2012?"

Other questions include, "Where does executive transportation rank on the FBI's priority list for aviation operations?"

The Justice Department did not immediately respond to a request for comment.